In order to explain the differences and confusion between the two names and clans, we need look at history, but that will even become shadowy in name variations.

The names MacTavish and MacThomas clearly stem from two variations of Thomas in the Gaelic, being, in order, Mac Tamhais, and Mac Thomaidh. MacTavish comes from a much older source in Donegal, Ireland being, MacGillesTamhais, The Servants of St. Thomas, who were the proprietors of Guill and Rosguill, of The Cenél nDuach, a branch of the Cenél Conaill. The Cenel nDuach was founded by the grandson of Niall of the Nine Hostages, Fergus Cennfota (Ferghus Ceannfaidh or Long Headed), also called Orc Doith (or Boars Fist), and also Tigernach Dui (Dark of Tigernach). We suspect at this time the O’Duins of Argyll are a branch of MacTamhais who used Dui (Dark - O’Duin) as a sirname, which later reverted to MacTavish with the birth of Tavis or Taus (fact-based research is continuing). The Irish annals often use one of the names to describe Fergus.

This Fergus Cennfota, married the daughter of Loarn Mhor, Son of Fergus Mor MacErc, first King of Scottish Dalriada. Her name was Eocha Loarn, and by her they had several sons, one of whom married Aethnea, a descendant of Cathor Mhor, and by her bore a son, Columb Kille, or Saint Columba. This is the MacTavish connection to Dalriada and Scotland, by blood and marriage. The connection is Royal. The MacTavish name is seen in various places with differing spellings or “local” pronunciations, in the Isles and the mainland of Scotland. Somewhere in the 7th century a host of the MacTavish left Donegal (Tir Connel), Ireland and made the trip to Dalriadic Scotland.

While I have not pinned down the exact time of this migration, it is seen in the MacTavish variations that erupt over time and locale. (What this migration and name explosion tells us is that the Traditional Campbell story of Tavis and Iver is highly suspect.) On the Isle of Man, for instance the name appears as Ma(c)Kewish, later shortened to Kewish, and in MacDonald-controlled-Argyll, under the Great Somerlead the name shows up as MacCauish, akin to MacCaush, MacCawis. All are locally spoken variations of MacTamhais and MacIlTavish, in a wide and varied dialectic state. As time worn on MacGillesTamhais, was contracted into further forms. MacGillesTamhais became MacGilhose, MacElhois, MacLawes, Macavish, Macommaish (the ‘c’ in Mac is often dropped), MacCause, MacThawish and so forth, but still with its foundation as a diminutive of Saint Thomas.

Some people think, and wrongly, that MacTavish is taken from the Braid Scots dialect, the name Tamas or Tammas, but this is read incorrectly. Tammas in Braid Scots means the same as Tamhais in Gaelic, confusing one for the other. Tamhais is a much older Irish (Erse) form, with Tamhus even older.

Some MacTavish variant spellings in Argyll and Perthshire also appear identical to names used of the Clan MacThomas, and this is where it can get confusing; in fact the name MacThomas appears in Argyll, but would be of Clan MacTavish. I have found no documentation to believe that members of Clan MacThomas migrated into Argyll. There does exist documentation that shows MacTavishes were settled in Perthshire.

We also find MacTavishes in Glasgow, as MacCaus, and in Fifeshire (etc..) as MacTawes and Tawes, as early as the 14th century, showing that our relatives did not stay in one place, but migrated over time into a vast area. MacTavishes are also found among the MacFarlanes having intermarried with them. We also find them co-existing with the MacThomases in the north. At a much later date it seems that the MacThomases also began a migration of their own.

The MacThomas from MacThomaidh (pronounced McHomy) appear to be of a different origin than are the MacTavishes, Clan MacThomas is a completely separate and distinct from MacTavish, and MacThomas of Finegand is their Chief, recognized as such by the Lord Lyon. The MacThomases trace themselves to "Great Thomas" a grandson of William MacKintosh the 8th Chief of Clan Chattan. Some name variants are Thom, Thomas, Thomason, and Thoms which claim origin from Tomaidh Mor, a descendant of the Clan Chattan Mackintoshes, regarded as the founder and first chief of the clan, and his grandson Aye (i.e. Adam), who as Aye MacAne MacThomas was a party to the Clan Chattan band of 2nd May 1543, is considered third chief. Roughly two generations later their chief dwelling is Glenshee, Perthshire, and from then on there is a reasonably unbroken line. In those days, of course, the language of the clan was the Gaelic, and the clan patronymic a diminutive of MacThomaidh (son of Tommy).

Now the confusion is even greater for there were MacTavishes in Perthshire at Glen Quaich as well as MacThomases, and it is reasonable to conclude that variations in the Gaelic tongue (Kintyre Gaelic is different from Scotland’s Northern Gaelic) lead to a conglomeration of Thomas name variations in that district. There is a possibility of intermarriage with MacThomases as well, since the two were neighbors both in Perthshire and elsewhere.

The MacTavish appear to be older than the MacThomas. The places where each clan could have co-mingled names and bloodline is in Glenshee and Glen Quaich, and the surrounding area in Perthshire, and possibly in Fife. These are areas to look at genealogically to determine clan affiliation if your family origin is from one of these places. Outside this area, and particularly in the east and south of Scotland, and the Isles, and as far afield as the City of Glasgow, and in Lothian and Dumfries, MacTavish surnames flourished. Either court, parish and/or commissariat records for most of these areas show a MacTavish presence as early as the late 1400s and early 1500s.

The MacThomas do not claim the name Thompson, and the added letter 'p' seems to be mostly a MacTavish variation.

The two clans appear quite distinct, but several name variations overlap.